Zajac was drafted 20th overall in 2004 by the Devils. He played two years of college hockey at the University of North Dakota, where current linemate Zach Parise was also his teammate. In his rookie season with New Jersey in 2006-07, Zajac scored 17 goals and had 42 points. Zajac has played with fellow youngster Zach Parise and veteran Jamie Langenbrunner for most of his career, as part of the “ZZ Pop Line.”